1. Field of the Invention
The present invention relates to a lead member for electronic parts, such as lead wire, which is used to connect various electronic parts, for example, semiconductor devices, to external circuits, and a process of producing such a lead member. More particularly, the present invention relates to an inexpensive lead member for electronic parts which has good heat resistance, good oxidation resistance and good corrosion resistance and thus is excellent in solderability, and a process of producing such a lead member.
2. Description of the Related Art
Various semiconductor devices such as diodes, transistors and ICs and various electronic parts, such as capacitors and resistors, incorporating such semiconductor devices have lead sections at which the devices or parts are connected to terminal sections of printed boards, for example. The lead section of an electronic part and the terminal section of a printed board are electrically connected to each other by lead members, such as lead wire as a typical example. The connection is usually achieved by soldering.
Conventionally, as such lead wire, a wire made of Cu alone or of a Cu alloy such as a Cu--Fe alloy or a Cu--Sn alloy, or a wire being a Cu-coated steel wire plated with Au, Ag, Sn, Ni or a Sn--Pb alloy is usually used.
The material used to make the lead wire is suitably selected depending on the electronic parts to which the lead wire is to be connected.
For electronic parts of which the major requirement is to maintain the mechanical strength after the connection, for example, a wire made of a Cu alloy or a Cu-coated steel wire is used. On the other hand, for electronic parts of which the major requirement is to secure the electrical conductivity, a Cu wire is used.
In the case of electronic parts which are subjected to etching during the manufacturing process, materials having corrosion resistance to the etchant used in the etching step are selected. Where electronic parts are subjected to heat treatment such as welding, soldering, cure molding or aging, materials having heat resistance and oxidation resistance are selected.
For example, a Si chip is subjected to etching using a strong acid or a strong alkali, and to connect a lead wire to the Si chip, the lead wire is soldered to the Si chip at a temperature as high as 350 to 400.degree. C. Further, when the resulting device is cure-molded using a silicone resin, it is applied with heat for curing in air at a temperature of 200 to 250.degree. C. In such a case, it is essential that the lead wire should have heat resistance, oxidation resistance and corrosion resistance.
As a lead wire for use with such a Si chip which is exposed to the aforementioned environmental conditions, a Ag-plated lead wire is currently known which includes a core made of oxygen-free copper containing Ag, a primary coat of Ni or a Ni alloy covering the surface of the core, and a Ag plate layer as a surface layer formed over the primary coat.
In this Ag-plated lead wire, Ag forming the surface layer has all of heat resistance, oxidation resistance and corrosion resistance, but Ag itself is expensive and migration is liable to occur. In addition, when the lead wire is heated at high temperatures in air, oxygen diffuses through the surface layer (Ag) and oxidizes the primary coat, with the result that the solderability lowers.
To solve the problems associated with the Ag-plated lead wire, a lead wire whose surface layer is made of Pd or a Pd alloy, instead of Ag, has been proposed (see Unexamined Japanese Patent Publication (KOKAI) No. 60-217693).
The surface layer of Pd or Pd alloy has excellent heat resistance, oxidation resistance and corrosion resistance and is also advantageous in that, unlike Ag, migration does not occur.
However, this conventional lead wire also is expensive because the material of the surface layer is Pd or a Pd alloy. To reduce the cost, the surface layer may be formed as thin as possible. If the surface layer is too thin, however, the solderability lowers at the time of heat treatment, as in the case of the Ag plate surface layer.
Thus, in order to enhance the solderability, the surface layer made of Pd or a Pd alloy must be increased in thickness, and this, however, leads to an increase in the cost of the lead wire.
The inventors hereof diligently conducted research to solve the problem of lowering of the solderability of the surface layer at the time of soldering and arrived at the following knowledges.
According to the knowledge of the inventors, when a lead wire is heated at the time of soldering, the metallic component constituting the surface region of the base of the lead wire, for example, the Cu component, thermally diffuses through grain boundaries of the metal forming the primary coat or through pinholes within the grain boundaries and contaminates the surface layer, and as a consequence, the solderability of the surface layer lowers.
Based on the above knowledge, the inventors gained the following technical concept; that is, if the grain size of the metal constituting the aforementioned primary coat grows larger to thereby reduce the number of the grain boundaries, then the thermal diffusion of the metallic component into the surface layer from the surface region of the base can be suppressed, so that the surface layer is prevented from being contaminated and the solderability thereof does not lower.
The present invention was created based on this technical concept.